1. Field of the Invention
The present invention relates to Class D amplifiers, and in particular, to an output signal reproduction circuit with zero phase delay.
2. Discussion of the Related Art
In a Class D amplifier, an input audio signal is transformed into an output signal capable of being reproduced into the original signal on an external load, usually a speaker. The basic operation of a Class D amplifier is shown in FIG. 1. An incoming analog signal AUDIO.sub.-- IN is converted by a pulse-width modulator, PWM 101, into a high-frequency rectangular wave DIG.sub.-- OUT, the average value of which tracks the original signal. Rectangular wave DIG.sub.-- OUT is fed into an output stage 102, which provides level shifting and splits the signal into a driving signal HI.sub.-- OUT and a driving signal LO.sub.-- OUT. Signal HI.sub.-- OUT and signal LO.sub.-- OUT are sent through output filters 103 and 104, respectively, to remove switching noise and provide an averaged output to drive a speaker 105. Output filters 103 and 104 are typically implemented as LC filters, as shown in FIG. 3a. An output filter 301 contains an inductor 302 and a capacitor 303 and acts as a low-pass filter on the signal going from output stage 102 to speaker 105.
In order to improve the accuracy of the output signal of a typical Class D amplifier, a feedback loop is used to monitor the output and provide slight adjustments as required. As shown in FIG. 2a, an example of this feedback loop includes a feedback amplifier circuit 206 and an error amplifier circuit 207. Feedback amplifier circuit 206 recombines the outputs of filters 103 and 104 into a single reference signal REF.sub.-- SIGNAL. Signal REF.sub.-- SIGNAL is then compared to the original signal AUDIO.sub.-- IN by an error amplifier circuit 207, which then generates an adjusted signal ADJ.sub.-- OUT that corrects for any discrepancies. Adjusted signal ADJ.sub.-- OUT is then processed normally by PWM 101, and the cycle continues. Another example of the feedback loop is shown in FIG. 2b. Two external feedback filters 203 and 204 read signals HI.sub.-- OUT and LO.sub.-- OUT, respectively, directly from output stage 102. Filters 203 and 204 typically contain an LC or RC circuit, as shown in FIG. 3b. In FIG. 3b, a feedback filter 304 includes a resistor 305 and a capacitor 306 arranged as a low-pass filter. Like filters 103 and 104, filters 203 and 204 remove switching noise and average the digital signals, and feedback amplifier circuit 206 combines their outputs to create signal REF.sub.-- SIGNAL for error amplifier circuit 207.
In the feedback loop implementations shown in FIGS. 2a and 2b, the accuracy of signal REF.sub.-- SIGNAL is critical in determining overall amplifier precision. If signal REF.sub.-- SIGNAL exhibits excessive distortion or phase delay, it cannot be validly compared to incoming signal AUDIO.sub.-- IN by error amplifier circuit 207, making error compensation useless or even detrimental. Distortion of signal REF.sub.-- SIGNAL is typically caused by switching noise from output stage 102 and back-EMF from speaker 105. Switching noise is a side effect of the normal operation of the power FET's in output stage 102, which are constantly cycling on and off in accordance with the output of PWM 101. This generates sharp current peaks and high-frequency noise that can couple to the basic output signal. Filters 103 and 104, and 203 and 204, are designed to remove switching noise by acting as low-pass filters, and often contain multiple filter stages to improve high-frequency rejection. However, the "clean" signal needed by error amplifier circuit 207 can impose a requirement for filter stages beyond those necessary for accurate sound reproduction at speaker 105. The additional filter stages add complexity and cost to the amplifier circuit, especially for the external filter implementation of FIG. 2b. However, the reason behind external filters 203 and 204 is to address the other potential source of signal distortion, back-EMF from speaker 105. Large displacements of the cone in speaker 105 can generate back-EMF in its voice coil, distorting the driving waveform. By decoupling external filters 203 and 204 from speaker 105, the effects of back-EMF are minimized, though at the cost of increased part count.
The other critical factor in amplifier precision is the phase delay of signal REF.sub.-- SIGNAL. If a phase difference exists between signals REF.sub.-- SIGNAL and AUDIO.sub.-- IN, the instantaneous magnitudes of the signals will differ even if the waveforms are identical in all other respects. This difference will be seen as an error by error amplifier circuit 207 and can lead to unwanted oscillations and distortions of the output signal as error amplifier circuit 207 attempts to "correct" a signal that may already match the original signal. Because of the discrete components in the filters that generate the output signals used by the feedback loop, phase shift is inevitable in conventional Class D amplifiers. The capacitances in filters 103, 104, 203, and 204 ensure a certain amount of phase delay in signal REF.sub.-- SIGNAL. This amount can be reduced by increasing filter capacitor size, but this increase also lowers the cut-off frequency of the filter and can have a detrimental effect on the frequency response of the amplifier.
Accordingly, it is desirable to provide a feedback amplifier circuit that is capable of providing accurate reproduction of amplifier output while introducing minimal phase shift.